the beauty of the garden today. "20/20" continues. Once again, Barbara Walters. Reporter: When the story about Elliot's shooting spree first broke, the videos and so-called "Manifesto" he left behind provided a rare glimpse into a mind broken by mental illness. Check it out. There's me. Reporter: At first glance, Elliot Rodger seemed like any other college age kid. Look at how fabulous I look. Reporter: Bragging about designer clothes. $300, Giorgio Armani. Reporter: Posing for selfies scored to retro pop. ? but the path Elliot was walking took him to the dark side of his mind. My problem is girls. I desire girls. But girls are not sexually attracted to me. It's not fair. It's not fair. Reporter: But why did those closest to him fail to see any warning signs when it seemed clear that Elliot was mentally disturbed? People have called your son "Evil." How would you describe Elliot? Elliot was far from evil. Something happened to him. I think he became very mentally ill. What I don't get is, we didn't see this coming at all. None of us. Reporter: But there were clues hidden in those early years that peter Rodger now admits he missed. The lonely young boy had grown into an introverted teenager. By the age of 13, he had walled himself into a cyber world. His constant companions, the characters in the video game "World of warcraft." In high school, Elliot was a loner. Bullied by his peers. They threw food at him during lunch and shoved him into lockers. I was an innocent, scared little boy trapped in a jungle full of malicious predators, and I was shown no mercy. He was afraid of boys and girls? He felt he had an inability to get along with them. And this is when we realized that he had a real fear of other human beings, of other kids his age. Reporter: Elliot fled from two high schools before finding refuge at a tiny 100-student school called Independence high. A school with a mission to help troubled children like the boy they called "Our Elliot." There was never a time in working with him or knowing him that I ever felt like his inability to interact socially would ever have led me to believe that this kind of rage was building inside of him. Never. Reporter: But the isolation hid a firestorm of frustration and anger. Its target during those teen years was sex. Finding out about sex is one of the things that truly destroyed my entire life. Sex, the very word fills me with hate. I would always covet it, I would always fantasize about it. But I would never get it. Reporter: Elliot had always been fascinated and repelled by sex. His first glimpse of pornography horrified him. I couldn't imagine human beings doing such things with each other. The sight was shocking and arousing. I was quite shaken for a few days. Did you talk to Elliot about sex? Yeah. What did you say? How it's so important to present yourself as a human being from the heart outwards, and not to be weird -- and I say weird because it is weird when you think girls are going to throw themselves at you. As a teenager, he described his interest in girls, but he said -- I'm quoting again -- "There is no way I could ever get them. And so my starvation began." Did he talk about this? He was always obsessive about this one issue. Obsessive about which issue? About not getting a girlfriend. Finding a girlfriend. Yeah. But here's the thing. According to this journal, and according to his acts, he was already planning heinous things in response to this emotional thing in his head that none of us could really see. Reporter: Elliot was tormented by the sight of pretty girls and public affection. Even his favorite haven at the Griffith park observatory became a reminder of failure. I don't understand you girls. It's like your sexual attraction is flawed. It's perverted. You're attracted to the wrong kind of guy. You should be attracted to guys like me, beautiful, magnificent guys. I saw sex as an evil and barbaric act, all because I was unable to have it. This was the major turning point. If I can't have it, I will destroy it. If Elliot had had the sex that he says he wanted so much, would things have been different? I don't think so because I think he had a condition. He had a very seriously -- advanced mental condition, and I think that whatever would have happened even if he got a girlfriend wouldn't have been able to have made him into the happy human being we wanted him to be. Did you take him to Las Vegas hoping that he would lose his virginity? I suggested to him that perhaps, you know, if his blocking point was having sex then perhaps, you know, in Las Vegas there might be an opportunity where it's legal for him to have sex. But he didn't want to go? He didn't want to go because he wanted to be loved for his heart. Retrospectively, I think it wouldn't have help at all. It wouldn't have? No, because retrospectively I realize how sick he was. But you know you can have these same things and not have another Elliot. You can have kids who -- Of course. -- Are frustrated and don't have girlfriends or boyfriends or -- you know and -- No, but when you put the combination together, there's a certain pattern. And I think it's important to talk about those patterns. And you missed it. Yeah. Yeah, too right I missed it. I wish I could turn the clock back for the -- for the victims. For our family. For all of those -- all the scars out there. Reporter: By the time Elliot reached his 18th birthday, the shy young boy had vanished, leaving only resentment and anger. Too terrified to approach young women, it was easier to hate them. That's a problem that I intend to rectify. I, in all my magnificence and power, I will not let this fly. It's an injustice that needs to be dealt with.

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